1. Field of the Invention
The present invention relates to safety-oriented controllers and, more particularly, to an apparatus for expanding a computation system with software for simulating a programmable logic controller for controlling safety critical applications.
2. Description of the Related Art
Hardware and software programmable logic controllers must be configured according to the requirements of the EN 61508 standard, such that they comply with functional safety requirements. In the case of safety-related systems, such as programmable logic controllers (PLCs) for critical processes, which contain electrical, electronic or programmable electronic components and the failure of which signifies a decisive risk to humans or the environment, these must be configured to particularly ensure safety. The following are exemplary applications which require an increased level of safety: nuclear power stations, control technology for systems of safety-related importance, railway applications, telecommunications technology, signaling and data processing systems, chemical processes or else, for example, small systems such as a punch for punching out sheet metal parts.
In addition to conventional programmable logic controllers, which are configured, for example, as a structural unit having a special system structure for controlling an industrial process, there are also software PLCs, i.e., programmable logic controllers which are simulated using software. Such a software PLC comprises, for example, a PC, preferably an industrial PC and, more particularly, a PC with input/output modules or a field bus expansion for connecting peripherals from the industrial process, such as over a Profibus or Profinet.
If such a software PLC is used to control one of the abovementioned safety-critical systems, the hardware and software that is used must comply with the safety requirements as set forth in the EN 61508 standard. This standard represents requirements for possible faults and failures which have to be detected or analyzed during operation to determine the proportion of non-critical failures. In order to achieve a high fault discovery level, it may be necessary for a second independent time base to be additionally available in addition to a system clock which is provided by a computation system. If the designer of a control system for an industrial process now wishes to use a standard PC with a software PLC, he is subject to hardware dependence because of different PC systems and is restricted in his use of a standard PC system with a corresponding programmable logic software controller.